Avidity-by-design: spatial control of T7 peptide presentation on polymersomes dictates blood-brain barrier transport fate

Targeting the transferrin receptor (TfR) holds promise for drug delivery across the blood-brain barrier (BBB), but conventional strategies suffer from competition with endogenous ligands and suboptimal trafficking. To overcome these limitations, we engineered a library of T7 peptide-functionalised pH-responsive polymersomes with precise control over ligand density and ligand insertion depth (δ) within a poly(ethylene glycol) (PEG) corona. We generated nanoparticles with δ values ranging from 0.3 (deeply inserted ligands) to 1.0 (fully exposed ligands), enabling systematic evaluation of how spatial ligand presentation influences BBB interactions. In vitro studies revealed that δ = 1.0 polymersomes exhibited efficient transcytosis, whereas δ = 0.6 polymersomes promoted endothelial retention, a divergence likely linked to differential receptor clustering and trafficking kinetics. Notably, TfR-mediated transport occurred independently of PACSIN2, distinguishing it from tubular transcytosis pathways observed with LRP1-targeted systems. These findings establish avidity-by-design as a strategy to tailor nanocarriers for either BBB penetration or vascular targeting, offering a modular platform for neurological therapeutics.